Multiple Cyberspy Groups Target Microsoft Exchange Servers via Zero-Day Flaws


Security researchers warn that multiple cyber-espionage groups are targeting the recently addressed zero-day vulnerabilities in Microsoft Exchange Server and say that more than 300 web shells have been identified on the compromised servers.

The issues (CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, CVE-2021-27065), which Microsoft addressed this week, were being abused as part of an attack chain that allowed for the execution of arbitrary code, remotely.

Microsoft said that state-sponsored Chinese hacking group HAFNIUM has been exploiting the vulnerabilities “in limited targeted attacks,” but new details shared by various security firms suggest broader targeting.

“ESET telemetry shows that (at least) CVE-2021-26855 is actively exploited in the wild by several cyber-espionage groups. Among them, we identified LuckyMouse, Tick, Calypso and a few additional yet-unclassified clusters,” ESET said on Twitter.

The company also revealed that, while most of the targets are located in the United States, attacks against servers in Europe, Asia and the Middle East have been identified as well. The assaults were aimed at government organizations, law firms, medical facilities, and private companies.

Organizations can determine whether they might have been compromised by looking in C:inetpubwwwrootaspnet_clientsystem_web for aspx files with names such as shell, supp0rt, aspnet, aspnet_client, and others, or for random filenames in the system_web subdirectory.

Managed detection and response (MDR) solutions provider Huntress says it has already observed more than 200 compromised Exchange Servers that received payloads within the “C:inetpubwwwrootaspnet_clientsystem_web” directory, and claims to have identified more than 350 web shells to date.

An analysis of approximately 2,000 Exchange servers has revealed that roughly 400 of them were vulnerable, with an additional 100 potentially vulnerable, Huntress reveals.

The targeted organizations, the security firm says, include “small hotels, an ice cream company, a kitchen appliance manufacture, multiple senior citizen communities and other ‘less than sexy’ mid-market businesses. We’ve also witnessed many city and county government victims, healthcare providers, banks/financial institutions, and several residential electricity providers.”

The large number of identified web shells, Huntress points out, suggests that multiple uncoordinated actors might have been involved in exploitation, or that automated deployment tools were used. The attacks were also able to bypass installed antivirus and EDR solutions.

“These attacks are grave due to the fact that every organization simply has to have email, and Microsoft Exchange is so widely used. These servers are typically publicly accessible on the open internet and they can be exploited remotely. These vulnerabilities can be leveraged to gain remote code execution and fully compromise the target,” Huntress also notes.

Given the critical nature of these vulnerabilities, organizations are advised to apply the available patches as soon as possible.

The U.S. Cybersecurity and Infrastructure Security Agency (CISA) has issued an alert on these vulnerabilities, and the Department of Homeland Security (DHS) has issued an emergency directive requiring agencies to look for indicators of compromise (IOCs) and either perform forensic investigations where compromise has been identified or apply the available patches where no IOCs were found.

Related: Microsoft Patches Critical SharePoint, Exchange Security Holes

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Ionut Arghire is an international correspondent for SecurityWeek.

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